In current paper a new thermodynamic model has been suggested for prediction of phase equilibria of binary and ternary systems containing polymer, salt and water. To calculate the activity coefficient of components three different terms of long range, middle range and short rang for excess Gibbs energy is considered. For long range forces between components a modified form of Debye-Huckel theory has been used. Moreover, to represent the middle range term of excess Gibbs energy a new expression of VERS model has been extended using the van der Waals volume parameter of the molecule. Also, UNIFAC-Freed-FV model has been employed for computation of short range part of activity coefficient To determine the binary interaction parameters of the model, 12 binary systems of water + PEG or salt and 11 ternary data set of water- PEG- Salt have been used. The regressed binary parameters have been applied for phase equilibria calculations of 54 aqueous two phase systems. It can be seen that the new model can predict the phase composition of ATPS very well. (C) 2016 Elsevier B.V. All rights reserved.